ALD deposition of ruthenium

ABSTRACT

A method to deposit nucleation problem free ruthenium by ALD. The nucleation problem free, relatively smooth ruthenium ALD film is deposited by the use of plasma-enhanced ALD of ruthenium underlay for consequent thermal ruthenium ALD layer. In addition, oxygen or nitrogen plasma treatments of SiO 2  or other dielectrics leads to uniform ALD ruthenium deposition. The method has application as a direct plating layer for a copper interconnect or metal gate structure for advanced CMOS devices.

FIELD OF THE INVENTION

This invention relates to electrical interconnection structures. Moreparticularly, it relates to “back end of the line” (BEOL)interconnections in high performance integrated circuits, and toadvanced CMOS device fabrication.

BACKGROUND OF THE INVENTION

Electrodeposition of copper is a standard deposition technique used forcopper interconnect applications. However, copper cannot beelectroplated directly onto diffusion barrier materials without a thincopper seed layer. In current processes, the copper seed layers aredeposited by Physical Vapor Deposition (PVD) for this purpose, often byderivative techniques of ionized PVD (I-PVD). However, in futuresemiconductor generations, a very conformal film deposition innanoscale, high aspect ratio structures will be required. This may onlybe achievable only by Atomic Layer Deposition (ALD) techniques. As analternative, copper electrodeposition can be also done on otherlow-resistance metal surfaces. The required material properties for thispurpose include nobility, formation of soluble or conducting oxides, andinsolubility in the copper bath. Preferably, direct plating materialshave good diffusion barrier properties as well as good adhesion todielectrics. A few metal layers have been identified as candidates,which are generally refractory metals such as Ru, Rh, Co, Mo, Cr, and W.

Recently, ruthenium is receiving attention as a directly plateablematerial due to its good properties as an electrode in DRAMapplications, as a metal gate for CMOS applications, and its applicationas a seed layer for direct plating of copper using an electroplatingprocess.

Ruthenoscene (or ruthenium cyclopentadienyl, Ru(C₅H₅)₂), otherwise knownas Ru(Cp)₂, has been used as a metal precursor which is reacted withmolecular oxygen to produce ruthenium thin films by ALD. Polycrystallineruthenium films with quite low resistivity (12–13 μΩcm) were obtainedwith low impurity levels. However, due to a nucleation problemassociated with the metal organic ruthenium precursor, only a verylimited, non-uniform deposition occurs on some dielectric surfaces,including silicon dioxide (SiO₂). To overcome this problem, the priorart used an in situ grown aluminum oxide (Al₂O₃) layer before ALD ofruthenium. There has been no know solution for direct deposition ofruthenium by ALD on SiO₂ and other dielectric surfaces. Even for CVD ofruthenium on oxides, it is a common practice to first deposit aruthenium seed layer by PVD. For the implementation of ruthenium by ALDto device processing, especially for direct plating and metal gatepurpose, some way of depositing metallic ruthenium films directly on todielectrics is essential.

Due to the RC delay in nanoscale integrated circuits, novel lowdielectric constant (low k) materials are being introduced. It has beenwidely known that vapor phase deposition including chemical vapordeposition (CVD) and ALD generally have nucleation problems on these lowk dielectrics. However, as the required film thickness of linermaterials, including direct plating liners, is getting thinner as thedevice scaling entering sub-100 nanometer technological node size, anucleation problem could be a potentially serious matter. Thus, varioussurface treatment technique to deposit metal thin films on dielectricswithout nucleation during the ALD of ruthenium is essential forimplementing ALD of metals as direct plating applications, as well asliner applications, in the BEOL area.

In another application of ruthenium ALD, the metal gate process requiredirect deposition of ruthenium on thin SiO₂ or high k materials.Ruthenium has been considered one of candidates for the metal gate ofdual gate CMOS devices due to its work function having the proper value.

SUMMARY OF THE INVENTION

It is therefore an aspect of the present invention to provide a methodfor forming a layer of ruthenium on a substrate which is free ofnucleation problems.

It is another object of the present invention to form a layer ofruthenium on a substrate which is has a low concentration of impuritiessuch as oxygen and carbon. A feature of the invention is the eliminationof nucleation issue during ruthenium film ALD on dielectric surfaces,such as SiO₂. The invention relates to the use of a nucleation aidinglayer deposited by plasma-enhanced ALD by using a ruthenium metalorganic precursor and atomic hydrogen. Once the underlayer of rutheniumfilm is formed, thermal ruthenium ALD using molecular oxygen instead ofhydrogen plasma is employed to deposit ruthenium with very low impuritylevels of carbon or oxygen.

After proper plasma treatment, the ruthenium metal films are depositedon SiO₂ without nucleation problems. The various surface treatmenttechniques, generally using plasma treatment prior to the atomic layerdeposition of ruthenium metal on dielectrics, results in rutheniumdeposition by ALD which is free of nucleation problems.

Thus, the invention is directed to a method for depositing ruthenium ona substrate, comprising exposing the substrate to a plasma which causesa high concentration of nucleation sites to be formed on the substrate,thus forming an exposed substrate; and depositing ruthenium on theexposed substrate by atomic layer deposition. The substrate is selectedor may be selected from the group consisting of silicon dioxide, methylsilsesquioxane, hydrogen silsesquioxane, low dielectric constantmaterials, and high dielectric constant oxide substrates.

The plasma is or may be an oxygen plasma, and may be generated bypassing molecular oxygen through a plasma generation source to produceactivated radicals to thereby generate a large number of nucleationsites on the substrate. The plasma may also be a nitrogen and may begenerated by passing molecular nitrogen through a plasma generationsource to produce activated radicals to thereby generate a large numberof nucleation sites on the substrate.

The atomic layer deposition may be performed by alternating steps ofexposing the substrate to a ruthenium precursor for a firstpredetermined period of time; and exposing the substrate to a plasma fora second predetermined time. The method further comprising evacuatingthe ruthenium precursor and the plasma between successive steps.

The ruthenium precursor is selected or may be selected from the groupconsisting of:

ruthenium cyclopentadienyl,

bis(ethylcyclopentadinyl))ruthenium); and

((2,4-dimethylpentadienyl)ethylcyclopentadienyl) ruthenium). Theruthenium precursor is carried in a carrier gas, preferably inert, suchas argon.

The substrate may be heated to a temperature of between 200 and 400° C.,and preferably, 350° C.

Using this method the ruthenium is deposited directly on the substratewithout use of a seed layer.

In accordance with another aspect of the invention, a method fordepositing ruthenium on a substrate, comprises performing plasmaenhanced atomic layer deposition of ruthenium on the substrate using aruthenium precursor and a plasma to form a thin film of ruthenium; anddepositing ruthenium on the thin film by thermal atomic layerdeposition. The plasma is preferably a hydrogen plasma. As set forthabove, the atomic layer deposition is performed by alternating steps ofexposing the substrate to a ruthenium precursor for a firstpredetermined period of time; and exposing the substrate to a plasma fora second predetermined time. The process may employ the parameters andconditions set forth in detail above.

In accordance with yet another aspect of the invention, a ruthenium filmformed by atomic layer deposition comprises less than three percentoxygen and less than 2% carbon. The film may be configured as a gate ofa CMOS device. The film may be deposited on a silicon dioxide substrate.The ruthenium film may be deposited directly on a substrate without useof a seed layer. The film may serve as a plating layer for a copperinterconnect.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features, and advantages of the presentinvention will become apparent upon further consideration of thefollowing detailed description of the invention when read in conjunctionwith the drawing figures, in which:

FIG. 1A illustrates deposited ruthenium on SiO₂ without any treatmentshowing macrosize defects.

FIG. 1B illustrates deposited ruthenium with oxygen plasma treatmentshowing defect free thin film deposition.

FIG. 2. illustrates x-ray diffraction data of ruthenium thermal ALDfilms with a PE-ALD ruthenium layer.

FIG. 3. is an atomic force microscope image of a ruthenium thermal ALDlayer with PE-ALD Ru layer on SiO2.

DESCRIPTION OF THE INVENTION

Variations described for the present invention can be realized in anycombination desirable for each particular application. Thus particularlimitations, and/or embodiment enhancements described herein, which mayhave particular advantages to the particular application need not beused for all applications. Also, it should be realized that not alllimitations need be implemented in methods, systems and/or apparatusincluding one or more concepts of the present invention.

An apparatus which may be used to perform the method in accordance withthe invention is described in the above mentioned paper entitledPlasma-Enhanced Atomic Layer Deposition of Ta and Ti For InterconnectDiffusion Barriers by S. M. Rossnagel, J. Vac. Sci. Technol. B18(4),July/August 2000. The teachings of this paper are incorporated herein byreference in their entirety.

Such noncommercial or a commercial atomic layer deposition (ALD) chambercan be used. Sample sizes as large as 200 mm diameter can be loaded andthe chamber can be pumped by a reactive-gas grade turbo molecular pumpwith a working base pressure of 10⁻⁷ Torr. The sample heating can bedone using a ceramic resistive heating plate, providing growthtemperatures up to 450° C., with the processes typically running at 350°C. The temperature can be controlled by varying current to the heater,which can be previously calibrated against a thermocouple attached tothe sample. Solid Ru(Cp)₂ (powder) contained in a glass tube can be usedas the metal precursor. Other metal organic Ru precursors includingRu(EtCp)₂(bis(ethylcyclopentadinyl)) ruthenium) orRu(OD)₃((2,4-dimethylpentadienyl) (ethylcyclopentadienyl)ruthenium)(also known as DER) can be used for the same purpose. The glass tube ismaintained at 80° C. to develop adequate vapor pressure and all thedelivery lines are heated to between 90–110° C. to prohibit condensationof the precursor. To improve the delivery, argon is used as a carriergas and the flow is controlled by a mass flow controller upstream fromthe source tube.

The RF plasma source, which includes a quartz tube wrapped with a coppercoil, can be used to produce plasma. Oxygen, nitrogen, and hydrogenflows are controlled by a leak valve or mass flow controller (MFC).

The deposition cycle includes the following steps: exposing thesubstrate to greater than 1,000 Langmuirs (a measure of the net flux ofgas atoms that impact a unit area) of Ru(Cp)₂ carried by argon gas,evacuating the chamber, opening the gate valve for the RF source and thegas valve for hydrogen for PE-ALD of ruthenium or the oxygen valve forthermal ALD of ruthenium, and shutting off the valves for evacuation. Nopurging gas is used between Ru(Cp)₂ and oxygen (or atomic H) exposure,but using a purging gas does not change the result. The films aredeposited on 5000 Å SiO₂ thermally grown on Si substrates. However, theinvention is not limited to a SiO₂ substrate, but includes various otherdielectric materials including SiCO, MSQ (methyl silsesquioxane), andHSQ (hydrogen silsesquioxane) and other low k materials, and high koxide substrates (those having a dielectric constant of greater than4.0, which is the dielectric constant of SiO₂). The film composition andthickness can be determined by Rutherford backscattering spectrometry(RBS). The microstructures are analyzed using X-ray diffraction (XRD)and morphology and roughness by atomic force microscopy (AFM).

The typical growth procedure includes 4 seconds of Ru(Cp)₂ exposure at 4sccm of flow rate, 2 seconds of pump out, 2 seconds of O₂ flow at 30sccm, and 2 seconds of pumping out. However, these conditions areprovided only by way of example and the invention is not be limited tothese specific process time. The growth temperature is typically 350°C., but growth temperatures of 200–400° C. are also useful. Further, ALDruthenium can be deposited at different flow rate of precursors. Atoxygen flow rate higher than 40 sccm, the deposited ruthenium film has aquite rough, milky surface. However, at low flow rate, a mirror likesmooth surface is obtained.

The thermal ALD of ruthenium on SiO₂ shows that deposition did not occureverywhere on the substrates and macrosize defects may be seen as inFIG. 1 a. This is evidence of the poor nucleation of ALD ruthenium onSiO₂ surfaces. RBS has shown that the carbon content is below thedetection limit of RBS (typically below 2%), and oxygen content is verylow; typically below 3%. The deposition rate is 1–1.1 A/cycle and theresistivity is 14–16 μΩcm.

EXAMPLE

To deposit nucleation problem free ALD ruthenium on SiO₂, the substratesare exposed to the plasma prior to the ALD of ruthenium for 10 minutesat the deposition temperature. Nitrogen, oxygen and hydrogen plasma maybe used at 500 Watts of plasma power. In general, hydrogen plasmaexposure does not, in and of itself, produce any significant improvementin terms of producing a uniform layer. However, oxygen plasma exposedSiO₂ produces very uniform, clean looking deposition on the substrates.Eight inch (20.3 cm) SiO₂ wafers are used for measurements ofuniformity. In terms of sheet resistance, the uniformity of thedeposited ruthenium films exhibits less than 5% variation in sheetresistance, without any bare surface spots. Similar improvement is alsoobtained by nitrogen exposure for 10 minutes on SiO₂ substrates. Theclean, macroscopic defect free ruthenium metal films deposited by ALD isshown in FIG. 1 b.

Comparison Example

PE-ALD using Ru(Cp)₂ and hydrogen plasma is attempted under the samedeposition conditions, as above. The deposited ruthenium layer thicknessis very small, and the sheet resistance is immeasurable (typicallyhaving a value greater than 1 Ωcm) even for 500 process cycles, thusindicating that any atomic hydrogen which may be present does noteffectively react with Ru(Cp)₂ adsorbed on the SiO₂ from the previouscycles. On the other hand, it appears that molecular oxygen oxidativelydissociates the ligands of metal precursors, producing a thin film byALD. However, the subsequent thermal ALD of ruthenium on a 100 cycledeposition of this very thin ruthenium PE-ALD film on SiO₂ shows uniformruthenium deposition, which is confirmed by electrical, sheet resistancemeasurements. Thus, the PE-ALD ruthenium provides a very thin rutheniumlayer for enabling uniform deposition of ruthenium film by thermal ALDon this very thin layer.

FIG. 2 shows the x-ray diffraction spectra of thin ruthenium filmsdeposited by thermal ALD for 300 cycles on the 100 cycle deposit ofruthenium by the PE-ALD process. The x-ray diffraction spectra showsthat the deposited film is hexagonal ruthenium metal film without anypeaks related to ruthenium oxide (RuO₂).

FIG. 3 shows the results of atomic force microscope measurements of thethermal ALD ruthenium film deposited on the PE-ALD ruthenium layer.There is no evidence of defects, indicating that the entire surface iscovered by ruthenium metal film evenly, as aided by the presence of thePE-ALD ruthenium underlayer.

Using this method, nucleation layer free ruthenium films are depositedon dielectric surfaces, thus having application as a direct platinglayer for copper interconnects. The PVD seed layer deposition ofruthenium prior to ruthenium ALD cannot be implemented for semiconductordevices with nanoscale via size, due to the limited conformality of thePVD process. However, by using the present invention, a thin conformallayer of ruthenium is deposited inside of the vias and trenches formedon various dielectrics. Copper electrodeposition can be performed evenlyon these ruthenium layers.

Direct deposition of ruthenium by ALD also is essential for thefabrication of dual work function metal gate CMOS devices. Ruthenium isone of the few metals having the proper work function for p-FET devicesand the deposition of ruthenium directly on gate oxide is veryimportant. In this case the use of a ruthenium PVD seed layer is hardlyuseful due to possible damage by the PVD process. The use of othermaterials as a nucleation aiding layer cannot be considered for thispurpose.

It is noted that the foregoing has outlined some of the more pertinentobjects and embodiments of the present invention. The concepts of thisinvention may be used for many applications. Thus, although thedescription is made for particular arrangements and methods, the intentand concept of the invention is suitable and applicable to otherarrangements and applications. It will be clear to those skilled in theart that other modifications to the disclosed embodiments can beeffected without departing from the spirit and scope of the invention.The described embodiments ought to be construed to be merelyillustrative of some of the more prominent features and applications ofthe invention. Other beneficial results can be realized by applying thedisclosed invention in a different manner or modifying the invention inways known to those familiar with the art. Thus, it should be understoodthat the embodiments has been provided as an example and not as alimitation. The scope of the invention is defined by the appendedclaims.

1. A method for depositing ruthenium on a substrate, comprising:exposing the substrate to a plasma which causes a high concentration ofnucleation sites to be formed on the substrate, thus forming an exposedsubstrate; and depositing ruthenium on the exposed substrate by atomiclayer deposition; wherein said atomic layer deposition is performed byalternating steps of: exposing the substrate to a ruthenium precursorfor a first predetermined period of time of four seconds; and exposingthe substrate to a plasma for a second predetermined time.
 2. The methodof claim 1, wherein the substrate is selected from the group consistingof silicon dioxide, methyl silsesquioxane, hydrogen silsesquioxane,other low dielectric constant materials, and high dielectric constantoxide substrates.
 3. The method of claim 1, wherein said plasma is anoxygen plasma.
 4. The method of claim 3, wherein the oxygen plasma isgenerated by passing molecular oxygen through a plasma generation sourceto produce activated radicals to thereby generate a large number ofnucleation sites on said substrate.
 5. The method of claim 1, whereinsaid plasma is a nitrogen plasma.
 6. The method of claim 5, wherein thenitrogen plasma is generated by passing molecular nitrogen through aplasma generation source to produce activated radicals to therebygenerate a large number of nucleation sites on said substrate.
 7. Themethod of claim 1, further comprising evacuating the ruthenium precursorand the plasma between successive steps.
 8. The method of claim 7,wherein the evacuating is done for a period of two seconds.
 9. Themethod of claim 1, wherein the ruthenium precursor is selected from thegroup consisting of: ruthenium cyclopentadienyl,bis(ethylcyclopentadinyl)) ruthenium); and((2,4-dimethylpentadienyl)ethylcyclopentadienyl) ruthenium).
 10. Themethod of claim 1, wherein the ruthenium precursor is carried in acarrier gas.
 11. The method of claim 10, wherein the carrier gas isargon.
 12. The method of claim 1, wherein said second predeterminedperiod of time is 2 seconds.
 13. The method of claim 1, wherein saidexposing of said substrate to said plasma is performed for 10 minutes orlonger.
 14. The method of claim 1, wherein said substrate is heated to atemperature of between 200 and 400° C.
 15. The method of claim 1,wherein said substrate is heated to a temperature of 350° C.
 16. Themethod of claim 1, wherein said ruthenium is deposited directly on saidsubstrate without use of a seed layer.
 17. A method for depositingruthenium on a substrate, comprising: performing plasma enhanced atomiclayer deposition of ruthenium on the substrate using a rutheniumprecursor and a plasma to form a thin film of ruthenium; and depositingruthenium on the thin film by thermal atomic layer deposition; whereinsaid atomic layer deposition is performed by alternating steps of:exposing the substrate to a ruthenium precursor for a firstpredetermined period of time of four seconds; and exposing the substrateto a plasma for a second predetermined time.
 18. The method of claim 17,wherein said plasma is a hydrogen plasma.
 19. The method of claim 17,further comprising evacuating the ruthenium precursor and the plasmabetween successive steps.
 20. The method of claim 19, wherein theevacuating is done for a period of two seconds.
 21. The method of claim17, wherein the ruthenium precursor is selected from the groupconsisting of: ruthenium cyclopentadienyl, bis(ethylcyclopentadinyl))ruthenium); and ((2,4-dimethylpentadienyl)ethylcyclopentadienyl)ruthenium).
 22. The method of claim 17, wherein the ruthenium precursoris carried in a carrier gas.
 23. The method of claim 22, wherein thecarrier gas is argon.
 24. The method of claim 17, wherein said secondpredetermined period of time is 2 seconds.
 25. The method of claim 17,wherein said substrate is heated to a temperature of between 200 and400° C.
 26. The method of claim 17, wherein said substrate is heated toa temperature of 350° C.
 27. A ruthenium film formed by the method ofclaim 1, comprising less than three percent oxygen and less than 2%carbon.
 28. The ruthenium film of claim 27, configured as a gate of aCMOS device.
 29. The ruthenium film of claim 27, deposited on a silicondioxide substrate.
 30. The ruthenium film of claim 27, depositeddirectly on a substrate without use of a seed layer.
 31. The rutheniumfilm of claim 27, for serving as a plating layer for a copperinterconnect.
 32. A method for depositing ruthenium on a substrate,comprising: exposing the substrate to an atomic hydrogen plasma whichcauses a high concentration of nucleation sites to be formed on thesubstrate, thus forming an exposed substrate; and depositing rutheniumon the exposed substrate by atomic layer deposition; wherein said atomiclayer deposition is performed by alternating steps of: exposing thesubstrate to a ruthenium precursor for a first predetermined period oftime; and exposing the substrate to molecular oxygen for a secondpredetermined time.
 33. The method of claim 32, wherein a nucleationaiding layer is formed by using a ruthenium metal organic precursor andsaid atomic hydrogen plasma.
 34. A ruthenium film formed by the methodof claim 33, comprising less than three percent oxygen and less than 2%carbon.
 35. The ruthenium film of claim 34, deposited on a silicondioxide substrate.